1. Field of the Invention
The present invention relates to a method and apparatus for controlling power consumption of stations on a carrier sense multiple access/collision avoidance (CSMA/CA)-based wireless local area network (LAN). More particularly, the present invention relates to a method and apparatus for controlling the power consumption of stations in a CSMA/CA-based wireless LAN system when the stations are in a standby mode for receiving a wireless signal from each other.
2. Description of the Related Art
Wireless local area network (LAN) systems based on the IEEE 802.11 standard employ carrier sense multiple access/collision avoidance (CSMA/CA) to share a wireless medium with one another. FIG. 1 illustrates a schematic diagram of a wireless LAN system based on the IEEE 802.11 standard. Referring to FIG. 1, the wireless LAN system includes a basic service set (BSS) 10, the Internet 11, and a content server 12.
The BSS 10 includes a wireless terminal 101 and an access point (AP) 102. The wireless terminal 101, which is a type of subscriber terminal requesting wireless LAN services, is called a station. The AP 102 is connected to a wired network, such as the Internet 11, or is wired, via a distribution system (DS), to another AP (not shown) that constitutes another BSS, allowing the AP 102 to bridge the Internet 11 and another network.
The station, i.e., the wireless terminal 101, has a hierarchical structure shown in FIG. 2. Referring to FIG. 2, the station includes a radio frequency (RF) layer 20, a baseband layer 21, and a media access control (MAC) layer 22. Here, the RF layer 20 and the baseband layer 21 are called physical layers.
The MAC layer 22 receives carrier sense information from the physical layers, i.e., the RF layer 20 and the baseband layer 21, and determines, based on the received carrier sense information, whether a wireless medium is currently being used before transmitting a frame to a receiving station. If the wireless medium is in an idle state, a sending station transmits the frame to the receiving station. All stations within a propagation range of the sending station can determine whether the wireless medium is being used when the frame is transmitted thereto by referring to a duration field value of the frame. Writing the time of use of the wireless medium into a frame and transmitting the frame is called virtual carrier sensing.
This process is illustrated in FIG. 3A. As can be seen in FIG. 3A, a receiving station A receives a data frame from a sending station B and transmits an acknowledgement (ACK) frame to the sending station B.
Since wireless LAN systems are mainly employed in portable or mobile devices, battery power is an important issue to be considered. Current wireless LAN systems consume about 80% or more of their energy while performing operations on the physical layers.
In order to minimize power consumption, stations have adopted a power management program allowing them to operate with reduced power consumption. The power management program is used in an infrastructure network using the AP 102. Power management of the stations can be carried out every predetermined number of beacon cycles using the power management program. Each beacon cycle lasts for an average of 100 msec. During the exchange of power management frames in a network, the AP 102 needs to buffer the power management frames, which makes it impossible to control power of the station at less than every 100 msec. Therefore, if a station consumes too much power during a real-time bi-directional service, e.g., a voice/video call, it cannot properly provide the real-time bi-directional service.
FIG. 3B is a timing diagram illustrating a virtual carrier sense operation performed by stations in a wireless LAN environment. Referring to FIG. 3B, first through third stations STA-1, STA-2, and STA-3 and other stations periodically perform virtual carrier sense operations to share a wireless medium. For example, when the first station STA-1 transmits (30) a data frame to the third station STA-3, the second station STA-2 and the other stations, which are not the target stations for the data frame, perform a virtual carrier sense operation (31). This virtual carrier sense operation (31) results in the second station STA-2 and the other stations consuming as much power as the third station STA-3. In response to the receipt of the data frame, the third station STA-3 transmits (32) an ACK frame, which indicates that no error has occurred in the received data frame, to the first station STA-1. The second station STA-2 and the other stations also perform a virtual carrier sense operation (33). Therefore, each of the second station STA-2 and the other stations unnecessarily consumes as much power as the first station STA-1 does.
Stations constituting a wireless LAN system consume power when transmitting and receiving frames to and from one another or carrying out a virtual carrier sense operation. Each of the stations consumes 30-50% more power when transmitting a frame than when receiving a frame. In addition, each of the stations consumes power only for transmitting a frame after performing a carrier sense operation. When performing the carrier sense operation, each of the stations consumes as much power as they consume for receiving a frame. When a plurality of stations shares a wireless medium, however, each station consumes more power when performing a virtual carrier sense operation than when transmitting a frame. For example, if twenty or more stations share a single wireless medium for a predetermined amount of time, they consume at least ten times more power when performing a virtual carrier sense operation than when transmitting a frame. Therefore, in a wireless network structure, such as a hot spot, in which a plurality of stations is served by one AP, the plurality of stations consume most of their power performing a virtual carrier sense operation. Consequently, it is necessary to reduce power consumption of the plurality of stations consumed when the plurality of stations perform a virtual carrier sense operation.